KR20160043990A - Al-COATED STEEL SHEET HAVING EXCELLENT TOTAL REFLECTION PROPERTIES AND CORROSION RESISTANCE, AND METHOD FOR MANUFACTURING SAME - Google Patents

Abstract

[PROBLEMS] To provide an Al-coated steel sheet with improved total reflection properties, corrosion resistance, and appearance when anodized.
[MEANS FOR SOLVING PROBLEMS] A steel sheet having an Al coating layer having an average thickness of 7 μm or more on the surface of a base steel sheet through an Al-Fe-Si alloy layer, wherein the average Si concentration in the surface layer portion from the surface of the Al coating layer to a depth of 3 μm is 2.0% Or less, preferably 1.3 mass% or less, and the area ratio of the Al-Fe intermetallic compound phase (compound phase) in the surface of the Al coating layer is 10% or less.

Description

TECHNICAL FIELD [0001] The present invention relates to an Al-coated steel sheet having excellent total reflection properties and corrosion resistance, and a method for producing the same.

The present invention relates to an Al-coated steel sheet which can be obtained by modifying a plated layer of a molten Al-based plated steel sheet by heat treatment, particularly exhibiting a high total reflectance and exhibiting good corrosion resistance.

Molten Al-based coated steel sheets are widely used for heat resistance applications. Most of the molten Al-based coated steel sheets used for practical use are produced by using an Al-based plating bath containing Si. By containing Si, the plating bath temperature can be lowered and the thickness of the brittle alloy layer (initial thickness of the alloy layer) formed between the base steel sheet (plating base plate) and the Al base plating layer can be made thin in the process of hot dip coating.

The alloy layer may grow even when the molten Al-based plated steel sheet is used for heat resistance. When durability at a high temperature is to be particularly improved, heat treatment (post-heating treatment) is performed after hot-dip coating, and a barrier layer of AlN is formed between the base steel sheet and the alloy layer. In this case, a base steel sheet containing N in an amount sufficient to form the AlN barrier layer is applied.

From the viewpoint of reducing the bath temperature of the Al-based plating bath, it is effective to use a plating bath composition containing about 7 to 12 mass% of Si. Most of the molten Al-based plated steel sheet contains 7 mass% or more of Si in the plating layer. However, some patent publications disclose examples in which a post-heating treatment is applied using a plating bath having a relatively low Si content of 6% or less (Patent Documents 1 to 9).

Japanese Patent Application Laid-Open No. 61-124558 Japanese Patent Application Laid-Open No. 3-104848 Japanese Patent Application Laid-Open No. 6-207262 Japanese Patent Application Laid-Open No. 6-330274 Japanese Patent Publication No. 3383119 Japanese Patent Application Laid-Open No. 8-319549 Japanese Patent Publication No. 3398810 Japanese Patent Publication No. 3485410 Japanese Patent Application Laid-Open No. 2000-290764

Since the molten Al-based plated steel sheet is applied to heat resistance applications as described above, it is desired to have good heat reflection characteristics. In addition, when applied to building materials such as tunnel walls and reflectors of lighting equipment, it is desired to have good reflection characteristics with little absorption of light. Such heat or light reflection performance depends largely on the total reflectance. Therefore, it is advantageous to have characteristics of high total reflectance in consideration of application to applications utilizing heat resistance and light reflection characteristics. In this specification, those having a high total reflectance property are expressed as " excellent in total reflection properties ".

In addition, in applications such as building materials, excellent corrosion resistance is also required. However, in the molten Al-based coated steel sheet produced in the plating bath containing Si, the corrosion resistance tends to be lower than that in the pure Al plating bath. Further, it is also assumed that the Al-based plated steel sheet is used by being subjected to anodizing treatment as in the case of the Al alloy material. However, the conventional Al-based plated steel sheet has a drawback that it is difficult to realize an anodized surface with good appearance due to the darkening of the appearance after the anodizing treatment.

An object of the present invention is to provide an Al-coated steel sheet which is superior to the conventional molten Al-based plated steel sheet in total reflection characteristics, corrosion resistance, and appearance in the case of anodizing treatment.

The above object is achieved by a steel sheet having an Al coating layer having an average thickness of 7 占 퐉 or more on the surface of a base steel sheet through an Al-Fe-Si base alloy layer, wherein the average Si concentration in the surface layer portion from the surface of the Al coating layer to a depth of 3 占 퐉 is 2.0% By weight or less, preferably 1.3% by mass or less, and the area ratio of the Al-Fe intermetallic compound phase (intermetallic compound phase) occupying 10% or less on the surface of the Al coating layer is 10% or less . The Al coating layer is obtained by modifying a molten Al-based plating layer containing Si by a heat treatment. At this time, the Si content of the hot dip coating bath is preferably 1.5% by mass or more and 6.0% by mass or less, more preferably 1.5% by mass or more and 3.0% by mass or less. But may be controlled in the range of 1.5% by mass or more and less than 3.0% by mass.

Here, the Al coating layer is a layer in which a matrix (base) is an Al phase. The Al coating layer may have an Al-Fe-based metal compound phase or Si phase.

The average Si concentration in the surface layer portion from the surface to the depth of 3 占 퐉 can be obtained by conducting EDX analysis (energy dispersive X-ray analysis) on a cross section parallel to the thickness direction of the Al coating layer. Specifically, assume a rectangular area of 3 mu m x 20 mu m having one side of 3 mu m in length in the thickness direction of the Al coating layer (i.e., the plate thickness direction of the steel sheet) in the SEM observation field of 5000 times magnification with respect to the cross section. A rectangular area in which all of the rectangular area is caught by the Al coating layer (that is, the rectangular area does not come out from the Al coating layer) and one side of 20 mu m in length is in contact with at least a part of the outermost surface of the Al coating layer is set as the measurement area. The average Si concentration (% mass conversion value) in the measurement region is determined by EDX analysis. The above measurement operations are carried out for randomly selected 5 or more visual fields and a value obtained by averaging the average Si concentration in each measurement region can be defined as " average Si concentration in the surface layer portion from the surface to 3 mu m in depth ".

The area ratio of the Al-Fe intermetallic compound phase on the surface of the Al coating layer is preferably such that the ratio of the area of the portion of the Al coating layer surface where the Al-Fe intermetallic compound phase occupies in the projected area of the observation region viewed in the plate thickness direction . The Al-Fe intermetallic compound phase that appears on the surface of the Al coating layer can be identified as a high phase in which the Fe content in terms of% by mass is followed by Al.

Further, in the present invention, as a method for producing an Al-coated steel sheet excellent in total reflection properties and corrosion resistance,

A step of producing a molten Al-based plated steel sheet having a plated layer of an average thickness of 7 탆 or more by using a molten Al-based plating bath having a Si content of 2.0% by mass or more and 6.0%

The molten Al-based plated steel sheet is heated and maintained at a temperature of 300 to 460 DEG C to spread diffusion of Si in the plated layer, and the plated layer is formed into an Al coating layer having an average Si concentration of 2.0 mass% A process for reforming

Is provided.

As a method for stably improving the total reflection characteristic and the corrosion resistance,

A step of producing a molten Al-based plated steel sheet having a plated layer having an average thickness of 7 탆 or more by using a molten Al-based plating bath having a Si content of 1.5% by mass or more and 3.0%

The molten Al-based coated steel sheet is heated and maintained at a temperature of 300 to 460 DEG C to spread diffusion of Si in the plated layer, and the plated layer is formed into an Al coating layer having an average Si concentration of 1.3 mass% A process for reforming

Is provided. In this case, the Si content of the molten Al-based plating bath may be controlled to be 1.5% by mass or more and less than 3.0% by mass.

According to the present invention, an Al-coated steel sheet excellent in the total reflectance of the surface compared to the conventional molten Al-based coated steel sheet, excellent in corrosion resistance, and excellent in appearance in the case of anodization is obtained. Particularly, since the total reflectance is high, it is excellent in heat reflection characteristics and light reflection characteristics, and is extremely useful in applications using heat resistance and reflection of light. This Al-coated steel sheet can be obtained by subjecting a molten Al-coated steel sheet, which can be manufactured by a general hot-dip plating line, to a post-heat treatment. Accordingly, the present invention contributes to the expansion of the use of the molten Al-based plated steel sheet.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram schematically showing a plated cross-sectional structure of a general molten Al-based plated steel sheet produced by using an Al-based plating bath having a high Si content.
Fig. 2 is a diagram schematically showing a cross-sectional structure of the plated steel sheet of Fig. 1 after post heat treatment. Fig.
[Fig. 3] A diagram schematically showing a plated cross-sectional structure of a molten Al-based plated steel sheet produced by using an Al-based plating bath having a low Si content.
Fig. 4 is a diagram schematically showing a cross-sectional structure according to the present invention after post-heat treatment of the plated steel sheet of Fig. 3;
5 is a diagram schematically showing a sectional structure of a molten Al-based plated steel sheet produced using a pure Al-based plating bath.
6 is a graph exemplifying the relationship between the heating temperature and the heating time required to make the average Si concentration in the surface layer portion from the surface of the Al coating layer 3 mu m deep to 2.0 mu m or less in mass.
7 is a photograph of a cross-sectional structure of a molten Al-based coated steel sheet produced by using a molten Al-based plating bath having a Si content of 9 mass% while being plated.
8 is a cross-sectional photograph of an Al-coated steel sheet obtained by subjecting a molten Al-based plated steel sheet produced by using a molten Al-based plating bath having a Si content of 9 mass% to post heat treatment in an atmosphere at 450 ° C for 24 hours.
9 is a photograph of a cross-sectional structure of a molten Al-based coated steel sheet produced by using a molten Al-based plating bath having a Si content of 2.5 mass% while being plated.
10 is a cross-sectional photograph of an Al-coated steel sheet obtained by subjecting a molten Al-based plated steel sheet produced by using a molten Al-based plating bath having an Si content of 2.5 mass% to post heat treatment in an atmosphere at 450 ° C for 24 hours.
11 is a SEM photograph of a portion of the alloy layer in a section while being plated of a molten Al-based plated steel sheet produced using a molten Al-based plating bath having a Si content of 2.5 mass%.
12] Fig. 12 is a graph showing the results of the post-heat treatment at 450 占 폚 for 24 hours in air on a molten Al-based plated steel sheet produced using a molten Al-based plating bath having an Si content of 2.5% by mass SEM picture.

The Al-coated steel sheet of the present invention can be realized by a method of modifying the plated layer of the molten Al-based coated steel sheet produced by using the hot-dip galvanizing bath containing Si by post heat treatment. However, in the post heat treatment, it is important to reduce the Si concentration in the surface layer portion of the plating layer by increasing the diffusion of Si in the plating layer to a larger extent than in the case of the conventional post heat treatment. In order to reduce the Si concentration in the surface layer portion, it is extremely effective to use a molten Al-based plating bath having a relatively low Si content.

Fig. 1 schematically shows a cross-sectional structure of a plated molten Al-based coated steel sheet produced by using a molten Al-based plating bath containing about 7 to 10 mass% of Si. The Al-based plating layer 3 is formed on the surface of the base steel sheet 1 as the original plate through the alloy layer 2. This alloy layer 2 is an " Al-Fe-Si alloy layer " having an intermetallic compound mainly composed of Al, Fe and Si. Al-based intermetallic compound phase 5 and Si phase 6 are present in the Al phase 4 which is a matrix (base) in the Al-based plating layer 3. The Al-Fe intermetallic compound phase 5 is distributed relatively near the alloy layer 2 and the Si phase 6 is distributed relatively near the surface 10.

Fig. 2 schematically shows a cross-sectional structure when the plated steel sheet shown in Fig. 1 is post-heated at a temperature of about 450 캜. The alloy layer 2 is slightly grown and thickened. The Si phase 6 existing in the plating layer 3 of Fig. 1 is spheroidized and is distributed in a large amount in the Al phase 4. In addition, the Al-Fe intermetallic compound phase 5 tends to slightly spheroidize. The Al coating layer derived from the plating layer after subjected to such post heat treatment is indicated by reference numeral (30).

In the case of a molten Al-based coated steel sheet produced by using a general molten Al-based plating bath containing Si, for example, at a content of 7 mass% or more, even if the post heat treatment is performed for a quite long time, 30, a large amount of the Si phase 6 remains. In the Al-coated steel sheet in such a tissue state, it was confirmed that the total reflection characteristic, the corrosion resistance and the appearance after the anodic oxidation were hardly improved with respect to the original coated steel sheet. That is, in the case of a general molten Al-based plated steel sheet, the above-mentioned characteristics of the post heat treatment can not be substantially improved.

Fig. 3 schematically shows the sectional structure of a plated molten Al-based coated steel sheet produced using a molten Al-based plating bath having a Si content of about 1.5 to 6.0 mass%. The alloy layer 2 present on the surface of the base steel sheet 1 tends to become slightly thicker than a general molten Al base coated steel sheet (Fig. 1) produced by a molten Al base plating bath having a high Si content. Is not a problem in ordinary use. This alloy layer 2 is mainly composed of an Al-Fe intermetallic compound or an Al-Fe-Si intermetallic compound as described later. Al-Fe intermetallic compound phase 5 and a small amount of Si phase 6 are observed in the Al phase 4 in the plating layer 3. The abundance of the Si phase 6 increases or decreases depending on the Si content in the plating bath. The Al-Fe intermetallic compound phase 5 is present near the alloy layer 2 and small near the surface 10. The Si phase 6 is mainly present near the surface 10.

In the molten Al-based coated steel sheet obtained by the molten Al-based plating bath having a relatively small Si content, since the amount of the Si phase 6 present is small, the general molten Al-based coated steel sheet (FIG. 1) Do. However, according to a study by the inventors, it is not possible to obtain a sufficient improvement effect on the total reflection characteristic, the corrosion resistance, and the appearance after the anodic oxidation only by such a state of the structure.

Fig. 4 schematically shows a cross-sectional structure that can be obtained when the post-heat treatment is performed on the plated steel sheet shown in Fig. 3 at a temperature of about 450 deg. C, for example, for about 24 hours for a relatively long time. There is no noticeable growth of the alloy layer (2). The presence of the Si phase is hardly confirmed in the Al coating layer 30 derived from the plating layer 3 in Fig. On the other hand, the shape of the Al-Fe intermetallic compound phase 5 does not change much. The average thickness of the Al coating layer 30 per section of the steel sheet needs to be 7 占 퐉 or more while sufficiently exhibiting heat resistance and corrosion resistance peculiar to the Al-based plating layer, and more preferably 20 占 퐉 or more. Although the upper limit is not particularly specified, it is usually within the range of an average thickness of 50 탆 or less and may be controlled to be 40 탆 or less.

According to a detailed study of the inventors, as shown in Fig. 3, the post-heat treatment of the plated steel sheet produced in the molten Al-based plating bath having a small Si content causes Si in the plated layer 3 to have a relatively low Si concentration And is trapped in the alloy layer 2. That is, it has been found that Si present in the plating layer 3 is used for the reaction of changing the alloy layer 2 to an Al-Fe-Si alloy layer mainly composed of an intermetallic compound having a higher Si content. By using this phenomenon, the Si concentration in the surface layer near the surface 10 of the Al coating layer 30 can be reduced. In addition, in the case of using the plating bath having a high Si content as shown in Fig. 1, the alloy layer 2 is an alloy layer mainly composed of an intermetallic compound having a high Si content from the beginning. Therefore, the phenomenon that the Si in the plating layer 3 is trapped in the alloy layer 2 by the post heat treatment does not occur so much.

When the Si phase is not observed as shown in FIG. 4 by the post heat treatment or the Si concentration of the surface layer near the surface 10 is sufficiently reduced particularly in the case of using the Al coating layer 30 having an extremely small amount of residual Si phase, It was found that the total reflectance and corrosion resistance of the Al coating layer were improved. Further, in order to improve the appearance after the anodizing treatment, it is important to reduce the Si concentration in the surface layer portion. Specifically, the total reflection characteristic and the corrosion resistance can be remarkably improved by setting the average Si concentration in the surface layer portion to 3 mu m deep from the surface to 2.0 mass% or less. By setting the average Si concentration in the surface layer portion to 1.3 mass% or less, more excellent total reflection characteristics and corrosion resistance can be realized. The lower limit of the average Si concentration in the surface layer portion from the surface of the Al coating layer to the depth of 3 占 퐉 is not particularly specified and may be reduced to 0% by mass. Considering the load of the post heat treatment process, % Or more.

The reason why the total reflection characteristic is improved by reducing the Si concentration in the surface layer portion is presumably because the Al purity of the plating surface layer portion is increased and reflection characteristics closer to pure Al can be given.

On the other hand, on the surface 10 of the Al coating layer 30, a portion where the Al-Fe intermetallic compound phase 5 is exposed is formed. It has been found that the Al-Fe intermetallic compound phase 5 present on the surface is a cause of deteriorating the appearance after the anodizing treatment. Further, it may be a factor for lowering total reflection characteristics and corrosion resistance. However, when a molten Al-based plating bath containing Si in an amount of 1.5 mass% or more is used, the Al-Fe intermetallic compound phase 5 tends to form near the alloy layer 2, The amount of the Al-Fe intermetallic compound phase (5) present is small. As a result of various investigations, it has been found that if the area ratio of the Al-Fe intermetallic compound occupied by the surface 10 of the Al coating layer 30 is suppressed to 10% or less, the Si concentration in the surface layer portion described above is reduced, The appearance can be remarkably improved. Also, the heat reflection characteristic and the corrosion resistance are improved. Regarding reduction of the area ratio of the Al-Fe intermetallic compound phase to 10% or less, the Si content in the molten Al-based plating bath can be controlled to 1.5% or more.

5 schematically shows a sectional structure of a molten Al-based plated steel sheet produced by using a pure Al plating bath. The alloy layer 2 formed between the base steel sheet 1 and the plating layer 3 can be obtained by using the Si-containing plating bath shown in Figs. 1 and 3 if the steel composition of the base steel sheet 1 The thickness is increased. The Al-Fe intermetallic compound phase 5 generated in the Al phase 4 as the matrix of the plating layer 3 is generated in large quantities in the vicinity of the surface 10 unlike the case of Figs. Even if the post heat treatment is performed, a large change in appearance is not caused in the cross-sectional structure. Therefore, in FIG. 5, reference numeral 30 corresponding to the Al coating layer after the post heat treatment is also shown.

The Al-Fe intermetallic compound phase 5 is exposed on the surface of the plated layer 3 in a molten Al-based plated steel sheet having a cross-sectional structure as shown in Fig. 5, which is produced by using a pure Al plating bath. The same applies to the case where the post heat treatment is performed. As described above, the Al-Fe intermetallic compound phase (5) present on the surface is a factor causing deterioration of appearance and corrosion resistance after anodic oxidation. In order to improve the appearance and corrosion resistance after the anodic oxidation, it is extremely effective to use a molten Al-based plating bath having a Si content of 1.5% by mass or more.

6 shows the average Si concentration in the surface layer portion from the surface of the Al coating layer to the depth of 3 占 퐉 for the case where the molten Al-based plating steel produced by using the molten Al-based plating bath having the Si content of 2.5 mass% Of not more than 2.0% by mass based on the heating time and the heating time. As described above, conventionally, the process itself of post-heating a molten Al-based plated steel sheet is known. However, in the conventional post heat treatment, it is difficult to sufficiently reduce the Si concentration in the surface layer portion of the Al coating layer as described above. In order to improve the total reflection characteristic, for example, as shown in Fig. 6, diffusion of Si, which is more qualitative, is required.

Figs. 7 to 10 illustrate photographs of cross-sectional structures before and after the post heat treatment.

7 is a photograph of a cross-sectional structure of a molten Al-based plated steel sheet produced by using a molten Al-based plating bath having a Si content of 9 mass% in a plated state. In the plated layer, an Al-Fe-based metal compound phase appearing as pale gray and an Si phase appearing black are dispersed in an Al phase which appears white.

8 is a cross-sectional structural photograph of an Al-coated steel sheet obtained by post-heat-treating the molten Al-based plated steel sheet using a molten Al-based plating bath having a Si content of 9 mass% in air at 450 캜 for 24 hours. In the Al coating layer derived from the plating layer, an Al-Fe-based metal compound phase appearing light gray and an Si phase appearing black are dispersed in an Al phase which appears whitish. These phases are spheroidized by heating.

9 is a cross-sectional micrograph of a molten Al-based plated steel sheet produced by using a molten Al-based plating bath having a Si content of 2.5 mass%. In the plating layer, an Al-Fe-based metal compound phase which appears to be pale gray is dispersed in an Al phase which appears white. There is also a darker visible Si phase. The amount of the Si phase decreases to a larger extent than in the case of Fig.

10 is a cross-sectional structural photograph of an Al-coated steel sheet obtained by subjecting a molten Al-based plated steel sheet produced by using a molten Al-based plating bath having an Si content of 2.5% by mass to post heat treatment in an atmosphere at 450 ° C for 24 hours. In the Al coating layer derived from the plating layer, an Al-Fe-based metal compound phase appearing as pale gray is observed in an Al phase which appears white. The existence of the Si phase can not be confirmed in this photograph.

Fig. 11 shows an SEM photograph of an alloy layer portion in a plated cross-section of a molten Al-based plated steel sheet produced by using a molten Al-based plating bath having a Si content of 2.5% by mass. Layer structure composed of the "upper layer" indicated by reference numeral 21 and the "lower layer" indicated by reference numeral 22 is shown as the alloy layer. Below the lower layer is the base steel. In the figure, the analysis positions of four points are represented by symbols a to d. Table 1 shows measurement results obtained by performing quantitative analysis by EDX on these four points.

It is presumed that the Si concentration is less than 3.0 mass% in the state where both of the upper and lower layers are plated, and the main intermetallic compound constituting these phases is an Al-Fe system as described in Table 1. [

12 shows the result of the post-heating treatment at 450 占 폚 for 24 hours in air on a molten Al-based plated steel sheet produced using a molten Al-based plating bath having a Si content of 2.5% by mass, ≪ / RTI > In the figure, the analysis positions of four points are represented by the symbols e to h. Table 2 shows measurement results obtained by performing quantitative analysis by EDX on these four points.

It can be seen that the Si content in the upper layer increased greatly by the post heat treatment. Si existing in the plating layer is trapped in the upper layer, and the upper layer shows a structure in which the structure is mainly composed of an Al-Fe-Si intermetallic compound.

[Base steel plate]

As the base steel sheet as the plating base, various types of steel which are conventionally applied to the molten Al base coated steel sheet can be applied. When used for heat resistance applications, it is preferable to apply a steel having an N content of 0.004 to 0.015 mass% in order to suppress the growth of the alloy layer. Specific examples of the steel component content include the following.

0.001 to 0.06% of Si, not more than 0.5% of Si, not more than 1.0% of Mn, not more than 0.016% of P, not more than 0.007% of S, not more than 0.012% of Al, not more than 0.015% of N, To 0.03%, the balance Fe and unavoidable impurities

The plate thickness of the plated original plate may be in the range of 0.1 to 3.5 mm, and may be controlled in the range of 0.2 to 1.6 mm.

[Al-based plating]

The molten Al-based coated steel sheet to which the present invention is applied can be produced by a general continuous hot-dip plating line. The plating bath composition is preferably an Al-based plating bath having a Si content of 1.5% by mass or more and 6.0% by mass or less. If the Si content of the bath is too high, it becomes difficult to sufficiently reduce the Si concentration in the surface layer portion by the post heat treatment in a later step. On the other hand, if the Si content of the bath is too low, the structure of the plating layer becomes closer to pure Al plating, and as shown in Fig. 5, the Al-Fe intermetallic compound phase 5 tends to form near the surface 10 , It is difficult to sufficiently reduce the Al-Fe intermetallic compound phase area ratio. It is more effective that the Si content of the bath is 1.5% by mass or more and 3.0% by mass or less. The upper limit of the Si content of the bath may be strictly controlled to be less than 3.0 mass%.

Fe usually enters the bath. The Fe content is preferably controlled to 3.0 mass% or less, more preferably 2.5 mass% or less. 1.0% by mass or less of B, 1.0% by mass or less of Zr, 1.0% by mass or less of Sr and 5.0% by mass or less of Mg, if necessary, as elements in other baths Maybe. Ti, B, and Zr are effective in improving the surface appearance due to miniaturization of the sequin size, Sr is effective for making the Si phase to be produced, and Mg is effective for improving the corrosion resistance. The remaining elements other than the above may be Al and inevitable impurities.

It is preferable that the plating adhesion amount is such that the thickness of the plating layer (excluding the alloy layer) per section is 7 占 퐉 or more, more preferably 20 占 퐉 or more. Although the upper limit is not particularly specified, it is usually within the range of an average thickness of 50 탆 or less and may be controlled to be 40 탆 or less.

[Post heat treatment]

Al-based plated steel sheet is subjected to heat treatment in order to obtain an Al coated layer having a low Si concentration in the surface layer portion. Since this is a heat treatment after hot-dip coating, this is referred to as " post heat treatment " in the present specification. As described above, in order to improve the total reflection property and the like, it is preferable to modify the Al coating layer having an average Si concentration of 2.0% by mass or less in the surface layer portion from the surface to a depth of 3 占 퐉, and more preferably 1.3% by mass or less.

As a result of various investigations, it has been found that, when the Al coating layer is modified with an Al coating layer having an average Si concentration of 2.0% by mass or less in the surface layer portion from the surface to a depth of 3 占 퐉, Molten Al-based coated steel sheet can be applied. And a molten Al-based plating bath having a Si content of more than 2.0% by mass and not more than 6.0% by mass may be used for the application.

In a more preferred embodiment, a molten Al-based plating bath having an Si content of 1.5% by mass or more and 3.0% by mass or less is used when the Al coating layer is modified to an Al coating layer having an average Si concentration of 1.3% It is effective to apply the molten Al-based coated steel sheet produced by the above method. And a molten Al-based plating bath having a Si content of 1.5% by mass or more and less than 3.0% by mass may be used as a target of application.

The heating temperature of the post heat treatment can be set in the range of 300 to 460 캜. And more preferably in the range of 380 to 460 ° C. If the heating temperature is too low, it becomes difficult to lower the surface layer of the plating layer to a low Si content. If the heating temperature is too high, excessive growth of the alloy layer tends to occur. The atmosphere is good.

The Si phase generated in the plating layer tends to be distributed in a large amount near the surface. In the post heat treatment, the Si on the Si layer is consumed for the high-Si reaction of the alloy layer, thereby reducing the Si content of the surface layer of the plating layer. It is essential to set the heating time so that the diffusion of Si in the plating layer and the high Si conversion reaction in the alloy layer proceed sufficiently. An appropriate heating time can be set by grasping the relationship between the heating temperature and the heating time sufficient to sufficiently attain the low Siization of the plating layer surface layer in advance according to the plating conditions (see FIG. 6). Further, even if the Si content in the plating bath is, for example, about 2.0% by mass, the average Si concentration in the surface layer portion from the surface to the depth of 3 占 퐉 in the plated layer (before the post heat treatment) is usually as high as about 2.5% by mass . Therefore, it is not sufficient to merely reduce the Si content in the plating bath, and it is possible to obtain an Al-coated steel sheet having the desired low-Si surface layer portion by carrying out a post heat treatment with qualities.

Example

A cold-rolled annealed steel sheet having the following chemical composition and having a thickness of 0.8 mm was prepared as a base steel sheet.

[Chemical Composition of Base Material Steel Sheet]

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.033% of C, less than 0.01% of Si, 0.23% of Mn, less than 0.01% of P, 0.013% of S, 0.01% of Al, 0.0027% An unavoidable impurity

The base steel sheet was used as a plated base plate to produce a molten Al-based plated steel sheet having an average thickness (here, the portion excluding the alloy layer) of about 30 to 50 탆 in the following plating conditions.

[Plating condition]

Si content in Al bath: listed in Table 3 and Table 4

Fe content in Al bath: about 2 mass%

Content of added elements other than Si and Fe in an Al bath: listed in Table 3 and Table 4

In the bath other than the above-mentioned elements, the components: Al and inevitable impurities

Plating bath temperature: 660 ° C

Dipping time in plating bath: 2 seconds

Average cooling rate to completion of solidification of plating layer: 13 ° C / sec

The obtained molten Al-based plated steel sheet was subjected to post heat treatment at the heating temperature and heating time described in Tables 3 and 4, and the following investigation was provided. The atmosphere of the post heat treatment was set to the atmosphere. For the purpose of comparison, a publicly available substrate without post heat treatment was also prepared.

[Measurement of average Si concentration to the depth of 3 mu m of the surface layer of the Al coating layer]

EDX analysis was performed on the cross section parallel to the plate thickness direction of the specimen by the following method. Assuming a rectangular area of 3 mu m x 20 mu m having one side of 3 mu m in length in the thickness direction of the Al coating layer in an SEM observation field of magnification of 5000 times magnification, all of the rectangular area is covered with the Al coating layer, (A mass% converted value) in the measurement area is obtained by EDX analysis is set as a rectangular area tangent to at least a part of the outermost surface And the average Si concentration in each measurement region was averaged to obtain an average Si concentration in the surface layer portion from the surface of the Al coating layer to a depth of 3 占 퐉.

[Measurement of surface occupied area ratio of Al-Fe intermetallic compound phase]

The surface area of the Al coating layer of the specimen was observed by SEM in the thickness direction and the area ratio of the Al-Fe intermetallic compound phase in the projected area of the observation region viewed in the thickness direction of the surface of the Al coating layer was obtained. The identification of the Al-Fe intermetallic compound phase showing the appearance on the surface can be carried out by EDX analysis. The area ratio was measured for five randomly selected fields, and the average value was adopted as the surface occupied area ratio (%) of the Al-Fe intermetallic compound phase.

[Measurement of average thickness of alloy layer]

The average thickness of the alloy layer was determined by observing a cross section parallel to the plate thickness direction of the specimen with an SEM. The alloy layer exhibited a phase structure of the upper and lower layers except for some examples using a plating bath having a high Si content.

[Measurement of average Si concentration in the upper layer of the alloy layer]

EDX measurement was carried out at a measurement point 10 points randomly selected near the center of the thickness of the upper layer as in the measurement points e and f shown in Fig. 12 in a cross section parallel to the plate thickness direction of the specimen, and the Si concentration was measured And the average Si concentration in the upper layer of the alloy layer. When the alloy layer had a single-layer structure, the average Si concentration at the center in the thickness direction was determined as a reference value.

[Evaluation of total reflection characteristics]

The total reflectance was measured on the surface of the Al coating layer of the sealing material. Using MPC3100 manufactured by Shimadzu Corporation, measurement was made under the conditions of a reflection angle of 8 degrees and a measurement wavelength of 550 nm, and the total reflection characteristics were evaluated according to the following criteria. ○ We judged more than evaluation to pass.

?: 75% or more of total reflectance

?: 70% or more and less than 75% of total reflectance

?: 65% or more and less than 70% of total reflectance

X: The total reflectance is less than 65%

[Evaluation of corrosion resistance]

The specimen was subjected to a wet test in which the specimen was held in an environment of 90 DEG C and 95% relative humidity for 500 hours to measure the rate of rust with the surface area of the rust, and the corrosion resistance was evaluated according to the following criteria. ○ We judged more than evaluation to pass.

◎: Less than 10% of rust

○: Rust occurrence rate 10% or more and less than 20%

△: Rate of rust occurrence 20% or more and less than 50%

×: Greater than or equal to 50% of rust

[Evaluation of Appearance after Anodizing Treatment]

The anode was subjected to anodizing treatment and the L value (brightness) of the obtained anodized surface was measured. The anodizing treatment conditions were as follows: treatment solution: sulfuric acid 150 g / L + aluminum sulfate 5 g / L, treatment temperature 25 캜, current density 5 A / dm ² , and treatment time 10 min. The appearance after the anodizing treatment was evaluated on the basis of the following criteria, and it was judged that ○ evaluation or more was acceptable.

?: L value of 90 or more

○: L value 88 or more and less than 90

?: L value 85 or more and less than 88

×: L value less than 85

[Evaluation of processability]

A sealing material was provided for cylindrical drawing processing and the state of peeling of the Al coating layer of the vertical wall portion of the workpiece was examined. The cylindrical drawing processing conditions were a drawing ratio of 2.0, a blank diameter of 80 mm, a diameter of 42 mm, a radius of 5 mm, a punch diameter of 40 mm and a radius of 5 mm. The processability was evaluated based on the following criteria, and ○ evaluation was judged as passing.

○: No peeling of the Al coating layer

X: In the peeling of the Al coating layer

[Evaluation of Surface Appearance]

The sequin fineness of the surface of the Al coating layer of the sealing material was evaluated by the span density. ○ We judged more than evaluation to pass.

◎: Spangle density 200 pieces / cm ² or more

?: Spangling density of 50 pieces / cm ² or more and less than 200 pieces / cm ²

X: Spangle density less than 50 pieces / cm ²

The results are shown in Tables 3 and 4.

In the examples of the present invention, the total reflection properties, the corrosion resistance, the appearance after the anodic oxidation treatment were improved, and the processability and surface appearance were good. Particularly, when the average Si concentration to the depth of 3 mu m of the surface layer of the Al coating layer was 1.3 mass% or less, it exhibited excellent total reflection characteristics, corrosion resistance, and appearance after the anodic oxidation treatment.

In contrast, No. 31 and No. 32, which are comparative examples, were produced by using a pure Al plating bath, and a large amount of an Al-Fe intermetallic compound phase was formed near the surface of the plating layer. This Al-Fe intermetallic compound phase remained almost unchanged even after the post-heating treatment (No. 32). In all of these, Si does not exist in the surface layer portion of the Al coating layer, but the surface occupied area ratio of the Al-Fe intermetallic compound phase is high, so that the total reflection property and the corrosion resistance are not improved and the appearance after the anodizing treatment is also bad. In addition, since the alloy layer is formed thick, the workability is bad.

Since the Si content in the plating bath No. 33 was too low, there was a tendency that a large amount of the Al-Fe intermetallic compound phase was generated near the surface of the plating layer. Therefore, each characteristic was inferior in the same manner as in the example using the pure Al plating bath. In this example, post heat treatment is not performed, but it is also difficult to reduce the surface area occupied area ratio of the Al-Fe intermetallic compound even by post heating.

Nos. 34, 37, 38, and 41 used a plating bath having a proper Si content, but the average Si concentration in the surface layer portion of the Al coating layer was high due to the post heat treatment being not performed or the heating conditions being inadequate. As a result, the total reflection characteristics and appearance after the anodic oxidation treatment were poor, and the improvement of the corrosion resistance was also insufficient.

Nos. 35 and 39 are molten Al-based coated steel sheets produced by using a plating bath having a high Si content, and the average Si concentration in the surface layer portion of the Al coating layer (the Al-based plating layer in these examples) The appearance after the anodizing treatment was bad.

Numerals 36 and 40 are obtained by post-heating the molten Al-based coated steel sheet produced by using the plating bath having a high Si content. However, since the average Si concentration in the surface layer portion of the Al coating layer could not be sufficiently reduced, the total reflection property, corrosion resistance, The appearance after treatment was not improved.

1 base plate
2 alloy layer
3 Al-based plating layer
4 Al phase
5 Al-Fe intermetallic compound phase
6 Si phase
10 surface
30 Al coating layer

Claims (7)

A steel sheet having an Al coating layer having an average thickness of 7 占 퐉 or more on the surface of a base steel sheet through an Al-Fe-Si base alloy layer, wherein the average Si concentration in the surface layer portion from the surface of the Al coating layer to a depth of 3 占 퐉 is 2.0% Wherein the area ratio of the Al-Fe intermetallic compound phase (intermetallic compound phase) occupying 10% or less on the surface of the Al coating layer is excellent in total reflection properties and corrosion resistance. A steel sheet having an Al coating layer having an average thickness of 7 占 퐉 or more on the surface of a base steel sheet through an Al-Fe-Si base alloy layer, wherein the average Si concentration in the surface layer portion from the surface of the Al coating layer to a depth of 3 占 퐉 is 1.3% Wherein the Al-Fe intermetallic compound occupies an area ratio of 10% or less on the surface of the Al coating layer, and is excellent in total reflection properties and corrosion resistance. The Al-coated steel sheet according to any one of claims 1 to 3, wherein the Al coating layer is obtained by modifying a molten Al-based plating layer containing Si by a heat treatment and having excellent total reflection properties and corrosion resistance. The aluminum coating layer according to claim 1, wherein the Al coating layer is formed by modifying a molten Al-based plating layer formed by a molten Al-based plating bath having a Si content of 2.0% by mass or more and 6.0% Al coated steel sheet. The aluminum coating layer according to claim 2, wherein the Al coating layer is formed by modifying a molten Al-based plating layer formed by a molten Al-based plating bath having a Si content of 1.5 mass% or more and 3.0 mass% or less by heat treatment, Al coated steel sheet. A step of producing a molten Al-based plated steel sheet having a plated layer of an average thickness of 7 탆 or more by using a molten Al-based plating bath having a Si content of 2.0% by mass or more and 6.0%
The molten Al-based coated steel sheet is heated and maintained at a temperature of 300 to 460 DEG C to promote the diffusion of Si in the plated layer to form the plated layer as an Al coating layer having an average Si concentration in the surface layer portion of 3 mu m deep from the surface to 2.0 mass% A process for reforming
Wherein the total thickness of the Al-coated steel sheet is in the range of 10 to 100 angstroms. A step of producing a molten Al-based plated steel sheet having a plated layer having an average thickness of 7 탆 or more by using a molten Al-based plating bath having a Si content of 1.5% by mass or more and 3.0%
The molten Al-based coated steel sheet is heated and maintained at a temperature of 300 to 460 DEG C to spread diffusion of Si in the plated layer to form the coated layer as an Al coating layer having an average Si concentration of 1.3 mass% A process for reforming
Wherein the total thickness of the Al-coated steel sheet is in the range of 10 to 100 angstroms.

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